#include<iostream>
#include<cmath>
#include<climits>

using namespace std;
int main(void) {
	int n,i;
	long long a=1,b=1,c=0;
	cin>>n;
	for(i=3;i<=n;i++)
	{
		c=a+b;
		a=b;
		b=c;
	}
	cout << c <<".00";
}

